I did almost the same, Maybe a little more lazy, didnt thought about hiding it . That was much smother , and a good idee. But however it works fine, even when you have to mount a "child-car-chair" ( hope you understand) . It feels pretty safe to , I have put the biggest metal washer between the upper support point and the bolt to reinforce it as much as I could.
In Sweden you dont have to use safety belt at cars 1968 and older . Dont know how it works in Denmark and Norwegian.